This invention relates to a method and apparatus for more accurately measuring and blending particulate material.
In the past, slide gate feeders were not very accurate at dispensing small amounts of material. The use of auger feeders was required in order to dispense small amounts with any accuracy. This was primarily due to the mechanical standard deviation of the slide gate. If a slide gate is repeatedly opened for a precise amount of time the amount of material that is dispensed through the gate varies. This variation is related to the amount of time that the gate is open. If a gate opens for a small amount of time (in milliseconds) a smaller error occurs than if the gate opens for a large amount of time (in seconds). This error is caused by the way that material flows through the hopper. Opening the gate for small amounts of time doesn""t give the material in the hopper a chance to move. Instead it slices the material present at the gate. When the gate opens for large amounts of time the material flows down the hopper. This flow is not perfect in any hopper due to the shape of the material and/or the fact that the material tends to surge in pulses. If a gate opens for several seconds, the material surges several times; sometimes it will surge three times, while other times it will surge two or four times. This variation causes significant error during metering. Typically, a control algorithm will keep metering until the appropriate target weight is reached; but this results in overshooting the target and therefore providing surplus material. The key is to prevent overshooting caused by this deviation. To compensate for those surges and overfills, some systems employ augers. An auger is unaffected by the material surging in the hopper because the material, in the auger system, is dispensed at the end away from the material surging from the hopper. Therefore, auger systems were the preferred method of metering small amounts. The problem with auger systems is that they have to be sized correctly. If an auger is too big for the material, then the resulting standard deviation will also be enormous and cause overshooting the target weight.
Additionally, when metering uses a gate feeder, the gate feeder""s cycle time (the time to open and close the gate) may prevent any material from being dispensed. When metering small amounts, the feed time is small and may be smaller than the cycle time. If the feed time is smaller than the cycle time, then no material will be dispensed. Consequently, if a control system designates a feed time for a gate feeder smaller than the cycle time, then the control system will mistakenly believe that material is being dispensed.
Since gate feeders do not have to be sized to correspond to different materials, it would be desirable to use gate feeders to feed material.
It is an object of the invention to provide a method and apparatus for accurately metering flowable, bulk solid material using a slide gate feeder that solves to an extent the above noted pitfalls by incorporating the standard deviation of a feeder into the metering process. In another aspect the invention incorporates consideration of a feeder""s cycle time into the metering process.
In another aspect, a blending system for proportionately combining materials method includes metering a feed material including the steps of: designating a recipe and a batch size, calculating target weight amounts of each ingredient material, calculating a corresponding feed time for each material feeder, and metering each feeder. The feed time is calculating using the feeder""s material flow rate, an initial target percentage of the target weight, which corresponds to an offset for the standard deviation of the feeder, and compensating for the feeder""s gate cycle time.